The grading of dielectric insulations for electrical cables for relative high voltage service comprising the introduction of predetermined gradations of dielectric characteristics in a body or unit of dielectric insulation enclosing an electrical conductor is an old concept and subject in the electrical art. For instance, various aspects and means of grading electrical insulation for cable are proposed and/or disclosed in a paper entitled "Silicone Rubber Graded Construction for High Voltage Insulation", by S. J. Nizinski, published in Wire and Wire Products, Volume 3, No. 5, May, 1972, page 628 et seq., and in British Pat. No. 1568 of 1901 and the following U.S. Pat. Nos.: 1,802,030, 2,123,746, 2,198,977, 3,160,703, 3,287,489, 3,433,891, 3,711,631, 3,816,639, 3,869,621.
The disclosures and contents of the foregoing publication and patents of the prior art are incorporated herein by reference.
The grading of electrical insulations for higher voltage service produces a more even and effective distribution or pattern of electrical stresses through the overall mass or body of the dielectric insulating material containing the electrical conductor in accordance with the electrical stress phenomena and mechanisms of the foregoing prior art. That is, the stress or voltage gradation is distributed or extended to increase its magnitude within the body of the insulating medium without any increase at the surface of the insulation adjacent to the electrical conductor by means of one or more incremental increases in the specific inductive capacitance values within the body of dielectric insulation in the direction of the conductor.
Grading of the specific inductive capacitance (or permittivity) values progressively through a body or unit of electrical insulation can be accomplished by any one of several means of the prior art. For example, grading of insulations has heretofore been accomplished by varying the density of the paper or sheet wrapping in electrical cables wrapped in oil impregnated insulations, the use of sections or components of two or more materials having different specific inductive capacitance values in a composite insulating body, or by the selective incorporation of fillers into dielectric insulating materials, or portions thereof, to modify or regulate the specific inductive capacitance values of the material, or sections thereof, as determined by the value of the specific inductive capacitance of a particular filler composition, blends of fillers, or the amounts thereof introduced into the dielectric material.
However, contemporary insulated cable manufacturing techniques comprising continuous extrusion molding of plastic polymeric insulating materials, such as polyolefin compounds, about the cable conductor while passing through the extrusion molding apparatus, are not as accurate or controllable with respect to the transverse symmetrical and/or concentric formation of the body of insulation thereabout as some former system such as wrapping or rolling paper or other insulating material in sheet form around the central conductor. Moreover, the degree or extent of a lack of symmetry and/or concentricity in cross-section of the extrusion formed insulated cable product is often accentuated with the current high speed plastic extrusion production operations and also with the sequential extrusion molding of multi-layers of components about the conductor in the formation of a composite body of plural layers of insulation surrounding the conductor for a graded cable.
Unsymmetrical, and/or non-concentric cross-sections of insulation in high voltage electrical cable having graded insulations are subject to uneven or disproportional electrical stresses, such as tangential stresses, at the interfaces between the adjoining components of the body of insulation having different dielectric properties or specific inductive capacitance values, which stresses in high voltage electrical transmission service of greater than about 69KV significantly contribute to or accelerate the breakdown of the insulation.